1. Field of the Invention
The present invention relates to a semiconductor sensor to detect a physical quantity such as acceleration, yaw rate, vibration, or the like.
2. Related Arts
A device with a sensing portion and processing-circuit portion formed monolithically on a single semiconductor substrate is priorly known as a sensor of this type. The sensing portion is formed by depositing a sacrificial film on the semiconductor substrate, depositing thereon a film which becomes a movable portion, and removing the sacrificial film, so as to suspend a movable portion in a movable state with respect to the substrate. An example of such a sensor is disclosed in U.S. Pat. No. 5,326,726. Additionally, this Patent Gazette proposes forming a bump on a rear surface of the movable portion, i.e., on a surface opposing the substrate. This is effective in preventing sudden collision of the movable portion and substrate; this bump contacts the substrate prior to the movable portion even if sudden force acts on the movable portion, and can restrict displacement of the movable portion to within a predetermined value.
Here, the bump is formed as follows. Namely, as shown in FIG. 11 of U.S. Pat. No. 5,326,726, after deposition of the sacrificial film, a divot is formed on the surface of the sacrificial film so as to be corresponding to the bump location. Thereafter, as shown in FIG. 12 of U.S. Pat. No. 5,326,726, an open portion which extends as far as the substrate surface is formed in the sacrificial film in correspondence with a position which becomes an anchor portion of the movable portion (beam structure), and after depositing a polysilicon film which becomes the movable portion, the sacrificial film is etched and constructs the beam structure.
That is to say, there exists a problem in which a plurality of patterning steps and mask steps, i.e., photolithographic processes, become necessary due to the need to form depressions (divot and anchor opening) of differing depth in the sacrificial layer.
Additionally, ordinarily wet etching is performed to remove the sacrificial layer. There exists a problem in that the substrate and movable portion adhere and become fixed at the bump portion due to surface tension of the etchant at this time.